Muon: The tiny sub particle disobeying laws of physics

Evidence is mounting that a tiny subatomic particle seems to be disobeying the known laws of physics, scientists announced Wednesday, a finding that would open a vast and tantalizing hole in our understanding of the universe.

The result, physicists say, suggests that there are forms of matter and energy vital to the nature and evolution of the cosmos that are not yet known to science.

“This is our Mars rover landing moment,” said Chris Polly, a physicist at the Fermi National Accelerator Laboratory, or Fermilab, in Batavia, Illinois, who has been working towards this finding for most of his career.

The particle célèbre is the muon, which is akin to an electron but far heavier and is an integral element of the cosmos. Polly and his colleagues — an international team of 200 physicists from seven countries — found that muons did not behave as predicted when shot through an intense magnetic field at Fermilab.

The aberrant behaviour poses a firm challenge to the Standard Model, the suite of equations that enumerates the fundamental particles in the universe (17, at last count) and how they interact.

“This is strong evidence that the muon is sensitive to something that is not in our best theory,” said Renee Fatemi, a physicist at the University of Kentucky.

The results, the first from an experiment called Muon g-2, agreed with similar experiments at the Brookhaven National Laboratory in 2001 that have teased physicists ever since.

At a virtual seminar and news conference Wednesday, Polly pointed to a graph displaying white space where the Fermilab findings deviated from the theoretical prediction. “We can say with fairly high confidence, there must be something contributing to this white space,” he said. “What monsters might be lurking there?”

“Today is an extraordinary day, long awaited not only by us but by the whole international physics community,” Graziano Venanzoni, a spokesperson for the collaboration and a physicist at the Italian National Institute for Nuclear Physics, said in a statement issued by Fermilab. The results are also being published in a set of papers submitted to several peer-reviewed journals.

The measurements have about one chance in 40,000 of being a fluke, the scientists reported, well short of the gold standard needed to claim an official discovery by physics standards. Promising signals disappear all the time in science, but more data are on the way. Wednesday’s results represent only 6 per cent of the total data the muon experiment is expected to garner in the coming years.

For decades, physicists have relied on and have been bound by the Standard Model, which successfully explains the results of high-energy particle experiments in places like CERN’s Large Hadron Collider. But the model leaves many deep questions about the universe unanswered.

Most physicists believe that a rich trove of new physics waits to be found, if only they could see deeper and further. The additional data from the Fermilab experiment could provide a major boost to scientists eager to build the next generation of expensive particle accelerators.

It might also lead, in time, to explanations for the kinds of cosmic mysteries that have long preoccupied our lonely species. What exactly is dark matter, the unseen stuff that astronomers say makes up one-quarter of the universe by mass? Indeed, why is there matter in the universe at all?

On Twitter, physicists responded to Wednesday's announcement with a mixture of enthusiasm and caution. “Of course the possibility exists that it’s new physics,” Sabine Hossenfelder, a physicist at the Frankfurt Institute for Advanced Study, said. “But I wouldn’t bet on it.”

Marcela Carena, head of theoretical physics at Fermilab, who was not part of the experiment, said, “I’m very excited. I feel like this tiny wobble may shake the foundations of what we thought we knew.”

Muons are an unlikely particle to hold centre stage in physics. Sometimes called “fat electrons,” they resemble the familiar elementary particles that power our batteries, lights and computers and whiz around the nuclei of atoms; they have a negative electrical charge, and they have a property called spin, which makes them behave like tiny magnets. But they are 207 times as massive as their better-known cousins. They are also unstable, decaying radioactively into electrons and super lightweight particles called neutrinos in 2.2 millionths of a second.

What part muons play in the overall pattern of the cosmos is still a puzzle.

Muons owe their current fame to a quirk of quantum mechanics, the non-intuitive rules that underlie the atomic realm.

Among other things, quantum theory holds that empty space is not really empty but is in fact boiling with “virtual” particles that flit in and out of existence.

“You might think that it’s possible for a particle to be alone in the world,” Polly said in a biographical statement posted by Fermilab. “But in fact, it’s not lonely at all. Because of the quantum world, we know every particle is surrounded by an entourage of other particles.”

This entourage influences the behaviour of existing particles, including a property of the muon called its magnetic moment, represented in equations by a factor called g. According to a formula derived in 1928 by Paul Dirac, the English theoretical physicist and a founder of quantum theory, the g factor of a lone muon should be 2.

But muons are not alone, so the formula must be corrected for the quantum buzz arising from all the other potential particles in the universe. That leads the factor g for the muon to be more than 2, hence the name of the experiment: Muon g-2.

The extent to which g-2 deviates from theoretical predictions is one indication of how much is still unknown about the universe — how many monsters, as Polly put it, are lurking in the dark for physicists to discover.

In 1998 physicists at Brookhaven, including Polly, who was then a graduate student, set out to explore this cosmic ignorance by actually measuring g-2 and comparing it to predictions.

In the experiment, an accelerator called the Alternating Gradient Synchrotron created beams of muons and sent them into a 50-foot-wide storage ring, a giant racetrack controlled by superconducting magnets.

The value of g they obtained disagreed with the Standard Model’s prediction by enough to excite the imaginations of physicists — but without enough certainty to claim a solid discovery. Moreover, experts could not agree on the Standard Model’s exact prediction, further muddying hopeful waters.

Lacking money to redo the experiment, Brookhaven retired the 50-foot muon storage ring in 2001. The universe was left hanging.

The big move

At Fermilab, a new campus devoted to studying muons was being built.

“That opened up a world of possibility,” Polly recalled in his biographical article. By this time, Polly was working at Fermilab; he urged the lab to redo the g-2 experiment there. They put him in charge.

To conduct the experiment, however, they needed the 50-foot magnet racetrack from Brookhaven. And so in 2013, the magnet went on a 3,200-mile odyssey, mostly by barge, down the Eastern Seaboard, around Florida and up the Mississippi River, then by truck across Illinois to Batavia, home of Fermilab.

The magnet resembled a flying saucer, and it drew attention as it was driven south across Long Island at 10 mph. “I walked along and talked to people about the science we were doing," Polly wrote. "It stayed over one night in a Costco parking lot. Well over a thousand people came out to see it and hear about the science.”

The experiment started up in 2018 with a more intense muon beam and the goal of compiling 20 times as much data as the Brookhaven version.

Meanwhile, in 2020 a group of 170 experts known as the Muon g-2 Theory Initiative published a new consensus value of the theoretical value of muon’s magnetic moment, based on three years of workshops and calculations using the Standard Model. That answer reinforced the original discrepancy reported by Brookhaven.

Reached by phone Monday, two days before the announcement, Aida X. El-Khadra, a physicist at the University of Illinois and a co-chair of the Muon g-2 Theory Initiative, said they had been waiting for this result for a long time.

“I have not had the feeling of sitting on hot coals before,” El-Khadra said.

On the day of the Fermilab announcement, another group, using a different technique known as a lattice calculation to compute the muon’s magnetic moment, got a different answer than El-Khadra's group, adding a new note of uncertainty to the proceedings.

“Yes, we claim that there is no discrepancy between the Standard Model and the Brookhaven result, no new physics,” Zoltan Fodor of Pennsylvania State University, one of the authors of a report published in Nature on Wednesday, said in an interview.

El-Khadra called it an “amazing calculation” but added that it needed to be checked against independent work from other groups.

Into the dark

The team had to accommodate another wrinkle. To avoid human bias — and to prevent any fudging — the experimenters engaged in a practice, called blinding, that is common to big experiments. In this case, the master clock that keeps track of the muons’ wobble had been set to a rate unknown to the researchers. The figure was sealed in envelopes locked in the offices at Fermilab and the University of Washington in Seattle.

In a ceremony on February 25 that was recorded on video and watched around the world on Zoom, Polly opened the Fermilab envelope, and David Hertzog from the University of Washington opened the Seattle envelope. The number inside was entered into a spreadsheet, providing a key to all the data, and the result popped out to a chorus of wows.

“That really led to a really exciting moment, because nobody on the collaboration knew the answer until the same moment,” said Saskia Charity, a Fermilab postdoctoral fellow who has been working remotely from Liverpool, England, during the pandemic.

There was pride that they had managed to perform such a hard measurement and then joy that the results matched those from Brookhaven.

“This seems to be a confirmation that Brookhaven was not a fluke,” Carena, the theorist, said. “They have a real chance to break the Standard Model.”

Physicists say the anomaly has given them ideas for how to search for new particles. Among them are particles lightweight enough to be within the grasp of the Large Hadron Collider or its projected successor. Indeed, some might already have been recorded but are so rare that they have not yet emerged from the blizzard of data recorded by the instrument.

Another candidate called the Z-prime could shed light on some puzzles in the Big Bang, according to Gordan Krnjaic, a cosmologist at Fermilab.

The g-2 result, he said in an email, could set the agenda for physics in the next generation. “If the central value of the observed anomaly stays fixed, the new particles can’t hide forever,” he said. “We will learn a great deal more about fundamental physics going forward.”